A liquid crystal display (LCD) is a passive type of display for providing a video image. LCDs are commonly used in television receivers, portable computer displays, and other electronic devices. An LCD usually requires a source of back light for operation because the LCD operates effectively as a light valve (thus it is referred to as a "passive" display device), allowing transmission of light in one state and blocking transmission of light in a second state. The typical LCD panel structure includes a liquid crystal polymer encapsulated between at least two planar glass plates in parallel with each other. A polarization layer is bonded to the outer surface of each glass plate such that the glass plates are sandwiched between two polarization layers.
The inner surface, or the surface facing the liquid crystal polymer, of each glass plate includes mutually perpendicularly oriented, conductive, transparent linear arrays of electrodes. The volume of the liquid crystal polymer between any two orthogonal arrays of electrodes forms a cube whose face area constitutes a pixel. The electrode arrays are connected on the periphery of the glass plates via input/output (I/O) strips for coupling to electronic circuitry for applying a voltage to the two sets of orthogonal conductive transparent electrode arrays. To one set of linear, parallel arrays of electrodes is provided video image information (typically to the vertically aligned electrodes), while to the other linear, parallel array of electrodes (typically the array oriented horizontally) are provided "ON" signals in a sequential manner such that each horizontal linear array of electrodes is sequentially turned on rendering the portion of the liquid crystal polymer under the turned-on electrode array transparent for presenting a portion of the video image. The vertically aligned electrodes are known as "signal" electrodes, while the horizontally aligned electrodes are commonly referred to as "scanning" electrodes.
The liquid crystal polymer is disposed intermediate first and second transparent electrodes. A voltage source couples the first and second transparent electrodes via a switch. Without a voltage applied across the liquid crystal polymer, the liquid crystal molecules are randomly oriented and incident light is randomly scattered by the liquid crystal polymer. With the switch closed and a voltage applied across the liquid crystal polymer, the liquid crystal molecules become optically aligned with the polarizer and render the "ON" line transparent. Thus, light incident upon this line is transmitted therethrough. In a conventional LCD display, a light source is placed behind the LCD panel to illuminate the whole panel and project a video image. The light source may be a fluorescent discharge tube or a metal halide or Xenon arc lamp. At any instant during operation, the LCD panel has only one horizontal line "ON," with only this line allowing for transmission of light through the panel. Facing electrodes adjacent all other horizontal lines in the LCD panel are OFF. Therefore, most of the light from the light source aft of the panel is blocked by the non-transmitting portions of the LCD panel and converted to heat. This arrangement is characterized by low light utilization efficiency and the generation of a substantial amount of heat which must be dissipated. Also, some of the light in the non-transmitting line areas is transmitted through the LCD panel by scattering which causes loss of contrast. In a standard NTSC system with 525 horizontal scan lines in a conventional projection LCD system, it can be seen that with only 1/525th of the input light utilized for producing a useful image, much energy is wasted in a conventional projection LCD system. This waste will increase in future systems employing high definition television (HDTV) displays which employ over 1000 horizontal scan lines and which will reduce light utilization efficiency even further. In order to increase image brightness and contrast, more powerful light sources are being developed. These more powerful light sources providing more lumens with more watts of power will produce even more heat and make component cooling and heat dissipation even more important design considerations.
This invention addresses the aforementioned limitations of the prior art by providing a backlighting source for an LCD panel which brightly illuminates only that portion of the LCD panel rendered transparent and containing video information and which maintains the backlighting beam on the transparent portion as it scans the display panel.